Breast density is a major cause of the failure to detect breast cancer with screening mammography in pre-menopausal women. Tamoxifen is used for breast cancer risk reduction and treatment, but little is known about its effect on breast density, or the correlation between tamoxifen-induced changes in sex steroids and symptoms which decrease quality of life and limit the use of tamoxifen. The ideal agent would be one which decreased breast density without producing systemic effects. We propose to address this problem using full-field digital mammography to measure breast density reproducibility. The specific aims are: 1) to correlate changes in breast density with the hormonal changes throughout the menstrual cycle using a salivary assay for sex steroids. This study will determine whether this is a significant factor which must be controlled for in intervention studies; 2) To evaluate the alterations in breast density produced by tamoxifen, and correlate changes in steroid hormone levels with changes in quality of life; and 3) to examine the use of a topically applied anti- estrogen, 4-hydroxytamoxifen, on breast density. The incidence of symptoms and changes in hormone levels will be assessed to determine the extent of systemic absorption and its clinical significance.. This study will provide useful information to women about the scheduling of mammographic examinations; determine if tamoxifen reduces breast density, another potential benefit of the drug for high-risk women; and ascertain whether a local strategy of density reduction is effective without producing systemic symptoms. If so, this would offer a major opportunity to improve our ability to detect breast cancer in the female population as a whole.